This invention relates generally to highlight color imaging and more particularly to an image creation method and apparatus wherein contrasting images are formed by selectively developing an electrostatic image with colored or otherwise distinctive toners.
It is common practice to add information to the face of a document or to highlight certain portions of it by underlining. It is also common to delete portions of the document either by crossing out information or by covering it with a blank piece of paper. As will be appreciated, writing data or underlining on the document spoils the original document while writing data or underlining on the copies requires much labor when many copies are required. Moreover, it is sometimes difficult to write on copies due to the impregnation of the paper substrate with silicone oil used in the fusing of the images to the substrate.
Recent developments in imaging systems have obviated the foregoing problems by the provision of methods and apparatus to reproduce an altered copy of the original document, as well as an identical copy thereof. Thus, recent innovations in printing machines provide for reproducing a document without unwanted information of the original document, and with the addition of new data thereto. In this way, the machine performs an editing function which significantly reduces the labor and time in preparing revised copies from the original document. Another editing function relates to highlighting an area of a document to be copied or printed in a color different from the rest of the document.
The latent image of an original document, formed by scanning the original document and projecting a light image thereof onto the charged portion of the photoconductive surface so as to selectively discharge the charge thereon, may be altered in various ways. The latent image may be edited by superimposing thereover an electrically modulated beam, such as a modulated laser beam, or the like. The modulated laser beam adds additional information or erases information from the scanned latent image. In this way, the resultant copy is altered from the original document. Various techniques have been devised for transmitting an electrical signal to modulate the laser so that the desired information is recorded on the latent image. The latent image may also be altered by selective actuation of light emitting diodes which are positioned perpendicular to the process direction of the printing machine.
The Panasonic E2S copier system uses an electronic pad to edit, board allows information recorded on a blackboard sized electronic board to be copied automatically by a copying machine on a copy sheet. In order to define the area that is to be altered, the coordinates of the relevant information on the original document to be modified must be transmitted to the printing machine.
The NP 3525 and Color Laser Copier manufactured by the Canon Corporation employs an edit pad which enables selected portions of a copy to be deleted. The NP 3525 and Color Laser Copier edit pad also permits color highlighting of designated areas of the document.
The formation of image areas to be highlighted is disclosed in U.S. Pat. No. 4,742,373. Highlighting in accordance with the disclosure of this patent is effected by using an editing pad to designate x,y coordinate values of information to be highlighted. The output from the editing pad is utilized to vary the intensity of a bank of light emitting diodes (LEDS) positioned perpendicular to the process direction of a charge retentive surface. Thus, for highlighting certain information of the original document, the LEDS are operated at half intensity. While the disclosure of this patent appears to be silent as to the actual method of developing such an image, it is customary to use two developer housings containing different color developers for this purpose which develop the electrostatic image at substantially less than the full contrast voltage.
For the purpose of creating optimum quality highlight color images in a single pass, it is desirable to use a scavengeless development system, at least in the second of the two developer housings employed. A scavengeless development system is one where the developer has minimal interaction with the toned images already formed on the charged retentive surface. Optimally, it would be advantageous if all interaction of developers with the image receiver could be avoided. A scavengeless development system is disclosed in U.S. Pat. No. 4,868,600 granted on Sep. 19, 1989 to Hays et al and assigned to the same assignee as this application. As described therein, toner is liberated from a donor roll by the application of an AC voltage to wires spaced from the donor roll by the toner thickness thereon. A DC bias applied across the gap between the donor roll and an image receiver controls development of the latent image by the liberated toner.
In the usual xerographic process, a bi-level electrostatic image is developed with a single color toner such as black toner. Multi-colored xerographic copies or prints prepared by the development of multiple bi-level electrostatic images require registered superposition of the developed images. Such multi-colored xerographic copies/prints derived from bi-level images can be made by using either several colored marking engines in tandem for single pass throughput or a single marking engine with multiple sequential colored imaging.
For a tri-level electrostatic image, highlight color printing can be obtained in a single pass with perfect registration. Since the black and color images are developed with opposite polarity toners, pre-transfer charging of the toner is required.
Pulsed voltage measurements with a scavengeless development system such as disclosed in U.S. Pat. No. 4,868,600 have shown that one can switch development on and off over a distance of only .about.0.5 mm on the image receiver. U.S. Pat. No. 4,913,348 granted to Dan A. Hays on Apr. 3, 1990 describes a spatially programmable development process whereby the rapid development switching of scavengeless colored development systems utilizing an AC biased wire enables the selective coloring of an electrostatic image in the direction parallel to the process. Such selective coloring is accomplished in a single pass of a charge retentive surface through various process stations.
Other devices capable of developing different colored images in a single pass in the direction parallel to the process direction are disclosed in various U.S. Patents as follows:
U.S. Pat. Nos. 4,710,016 and 4,754,301 disclose imaging apparatuses which utilize two colored developer housings which are adapted to be selectively moved between development and nondevelopment positions relative to the charge retentive surface.
U.S. Pat. No. 4,752,802 illustrates a magnetic brush development system designed so that toner or developer can be withdrawn from the development zone without having to move the developer housing away from the charge retentive surface as required in the '301 patent. Two developer units are employed and are selectively used for each copying operation by the operator manipulating a selector switch provided on a control panel. At least one developing unit of the two component magnetic brush type is disposed opposite an electrostatic latent image receiver. The developing units have a developing sleeve in which is housed a magnetic core assembly that can be oriented by a drive means to switch development on and off by controlling the height of the developer in the development zone and the amount of developer metered onto the roll. The rotatabe developing sleeve is turned on and off simultaneously with the magnet orientation to switch development on and off, respectively. For development, the magnetic core assembly is so rotated that a weak magnetic or non-magnetic portion is at a position opposite to a level regulating member, and a high magnetic field is at a position opposite to the electrostatic latent image carrier. Furthermore, the rotating sleeve is stopped when development is switched off. Thus, to switch off development, the developing powder present on the outer periphery of the developing sleeve is shunted away from the developing zone and the sleeve rotation stopped. Such shunting of the developing powder is carried out with any of the developing units other than one selected for developing. Since development is obtained with a strong magnetic field in a zone adjacent to the electrostatic latent image carrier, the transitional width for switching color development is .about.8 mm. This implies that information separated by less than 8 mm in the process direction cannot be color separated by this process.
U.S. Pat. No. 4,811,046 granted on Mar. 7, 1989 to Jerome E. Mays and assigned to the same assignee as this application discloses a tri-level image development system comprising two developer housings, each containing at least two magnetic brush developer rolls. The developer rolls in one of the housings are adapted to be reverse rotated for the purpose of removing toner material from the development zone formed by the two rolls and a charge retentive surface.
While not specifically related to color imaging, U.S. Pat. No. 4,568,955 issued on Feb. 4, 1986 to Hosoya et al may be relevant to other aspects of the present invention. This patent discloses a recording apparatus wherein a visible image based on image information is formed on an ordinary sheet by a developer. The recording apparatus comprises a developing roller spaced at a predetermined distance from and facing the ordinary sheet and carrying the developer thereon. It further comprises a plurality of addressable recording electrodes positioned behind the ordinary sheet and connected to signal sources for attracting the developer on the developing roller to the ordinary sheet by generating an electric field between the ordinary sheet and the developing roller according to the image information. A plurality of mutually insulated electrodes are provided on an insulative developing roller and extend therefrom in one direction. AC and DC voltage sources are connected to the electrodes, for generating alternating electric fringe fields between adjacent ones of the electrodes to cause oscillations of the developer positioned between the adjacent electrodes along electric lines of force therebetween to thereby liberate the developer from the developing roller.
As will be appreciated, selective coloring in a direction perpendicular to the process direction together with coloring in a direction parallel to the process direction is highly desirable.